I'm interested in using some rotary tool and die grinder bits, wheels, and brushes on my bench grinder. Since they are on mandrels already or need arbors 1/4 inch or less I can't use them this way. Mounting a drill chuck seems like a grand idea (other than some uncertainty about speeds), but the grinder shaft is threaded M12-1.75 and common chucks are 1/2-20. I have a 3" mini grinder that might also work well but it has a 3/8-16 thread rather than the 3/8-24 used for smaller drill chucks.

Are there good sources for threaded adapters that would be useful for this? I haven't found any yet that do the job or are close enough to modify for my purpose. I have considered making my own but do not have a lathe and am not convinced I can get close enough with my tools to keep things tight and minimize runout. I have even considered drilling out the threads on a 3/8-24 chuck and re-tapping for 1/2-20.

Any links to related pages or thoughts on this would be much appreciated.

Welcome to the forum. I can't answer your question specifically and am not sure what you are exactly doing with the various things you want to mount on your grinder, but I will mention this...some bench grinders turn 1750 RPM while some turn 3450 RPM. IMHO, neither of these speed come close to the RPM needed to effectively run a small bit designed for a Foredom or Dremel type tool. These are small diameter tools that are designed for high RPM to work properly. Again, I don't know your application so this may not be the case.

Monte, It would make little sense to make/buy adapters for a bench grindle when you can go buy a purpose made Dremel/clone for little money. Look at ebay. Now you have a 25000 rpm grinder that will do some work. Since you have not said what you inted to mill or drill it may not help as well.

Monte. This occurred to me a long time ago and to this day I have a grinder with a fair coarse wheel to the left, a face grind green wheel to the right with a Jacobs chuck in the center.
1/2x20 was pretty standard then with bench grinders. The speed is nowhere near the operating speed of the small units but at 3450 rpm they do work. Safety glasses are good yet so far nothing has let loose. The mini wheels are too small yet there are times I can go right from finishing whatever to that little wheel. My chuck was origionally a crappy one but I found a good quality from a destroyed drill worked fine. The intermediate sizes with 1/4'' shanks are good for chip breakers or minute chamfers without messing up a larger wheel.
There are of course shields which swing and rests which swing but with these at that speed, it's pretty much freehand. You just have to remember that pressure cannot makeup for cutting speed.

I dont think there is any way without a lathe to make an adaptor or to redrill and tap a chuck that will not produce wobbly result. One suggestion is that they sell small grinders with a flex shaft out one end. And look out for a real old 6 inch grinder they all used to have 1/2 20 threads. They also used to sell threaded addaptors to adapt a radial arm saw to accept a 1/4 inch router bit. 1/4 inch collet . I would also try a lapadary supply store because i am pretty sure they sell grinders with a small wheel out one side and a 1/4 inch collet out the other side.

My primary objective is to bench mount some 2"-4" unitized wheels that have 1/4" arbor holes for deburring cut items with internal edges that don't seem to be reached by larger radius wheels. My initial attempts with smaller wheels have been successful, but I thought it would be more convenient to have them always available and sometimes it is easier to handle the pieces than the die grinder. Mounting bits and abrasives on smaller mandrels is not really important, and the 3M Radial Bristles can go on the 3" as is, though a small bushing might be needed to keep them cleanly centered.

My 3" grinder is variable up to 10,000 and I can mount it so that 4" is possible. I grabbed a 3/8-16 to 3/8-24 stud today at an auto parts store and a 3/8-16 coupling nut from Home Depot. Everything connects, but the slop in the combined assembly allows about 0.5mm wobble. It seems that the parts are all made assuming that the other parts will all be oversized. I'll tap a standoff 3/8-16 all the way through and see if I can improve on this.

I will also follow-up on a number of the suggestions given. Mounting the die grinder crossed my mind also so I will look for options better than just crunching it in a vise like my first attempt.

Thanks again for all the thoughts and advice so far, and I look forward to reading more.

spro wrote:I have a grinder with a fair coarse wheel to the left, a face grind green wheel to the right with a Jacobs chuck in the center.

Bro spro, I admit I sometimes find your verbocity a bit challenging but, I gotta see the two wheeled grinder with a chuck in the middle. Can you post a pic?

Stevec, I did a double take also. Then I reread Spro's post and figured he meant the chuck was in the center of the green wheel, not in the center of the arbor. I am sure Spro will correct me if I am wrong.

I have a shop made spindle with JT4 on one end, and a 3/4" arbor on the other. It's on a stand alone mount that is mounted when needed and driven by a 2 hp 3ph motor with VFD. The arbor generally gets a heavy knotted wire brush for heavy cleaning and paint removal. The other side has an old N18 that I rebuilt with new jaws/nut and use for mounting large burs or abrasive cartridges, even large mounted stones. Basically, the N18 works like a quick mount.

I have a number of grinders just out of convenience and what I've found along the way for good deals (I love my 7" Baldor). But if I didn't want a bench grinder to be a bench grinder, then I would start with a free mounted arbor with separate motor and take it where ever I wanted to go. And the best part is, beside the greater flexibility, is that the motor isn't always getting in the way. Plus you can use pulleys to alter mechanical reduction, or run it faster if that is what is needed...

The flex drive shaft the Steve shows is the same idea I have. Years ago someone gave me a flex shaft with a collect holder. Then I found a 10,000rpm motor to run it with. Works good and more power than a air grinder.